1. Field of the Invention
The present invention relates to an image pickup signal processing apparatus for performing nonlinear processing such as .lambda.-correction and white compression processing.
2. Related Background Art
A conventional apparatus of this type is known and used in a variety of applications. Of these conventional applications, an apparatus having good frequency characteristics in nonlinear processing is shown in FIG. 4.
This processing apparatus comprises a VCA (Voltage-Controlled Amplifier) 31, amplifiers 32 and 35, a clamp circuit 33, a nonlinear processor 34, and an A/D (analog/digital) converter 36. The nonlinear processor 34 includes pnp transistors Q.sub.1 to Q.sub.7 and resistors R.sub.1 to R.sub.12.
An operation of this processing apparatus will be described below.
A video input signal is amplified to an optimal signal level by the VCA 31 controlled by a gain control signal. The VCA 31 is arranged to correct the white balance. The level-controlled gain control signal is input to and amplified by the amplifier 32. The amplified signal is input to the clamp circuit 33, and the black level of the video signal is fixed to a predetermined potential. The clamped video signal is input to a buffer amplifier and a differential switch in the nonlinear processor 34. The buffer amplifier is constituted by the transistor Q.sub.1 and the resistor R.sub.1. The differential switch is constituted by a set of the transistors Q.sub.2 and Q.sub.3 and the resistor R.sub.2, a set of the transistors Q.sub.4 and Q.sub.5 and the resistor R.sub.3, and a set of the transistors Q.sub.6 and Q.sub.7 and the resistor R.sub.4. The bases of the transistors Q.sub.3, Q.sub.5, and Q.sub.7 receive reference voltages V.sub.A, V.sub.B, and V.sub.C obtained by dividing a power source voltage V.sub.CC through the resistors R.sub.5 to R.sub.8. A clamp voltage V.sub.CP and the above voltages V.sub.A, V.sub.B, V.sub.C, and V.sub.CC satisfy the condition V.sub.CP &lt;V.sub.A &lt;V.sub.B &lt;V.sub.C &lt;V.sub.CC.
When the voltage of the video signal falls within the range between the voltages V.sub.A and V.sub.B, the transistors Q.sub.2, Q.sub.4, and Q.sub.7 are turned on, and the transistors Q.sub.3, Q.sub.5, and Q.sub.6 turned off. The level of the video signal input is attenuated to R.sub.12 /(R.sub.9 //R.sub.10 //R.sub.11 +R.sub.12) (where // represents a resistance of parallel-connected resistors) by the resistors R.sub.9 to R.sub.11.
When the voltage of the video signal falls within the range between the voltages V.sub.CP and V.sub.A, the level of the input video signal is attenuated to 1. When the voltage of the video signal falls within the range of the voltages V.sub.B and V.sub.C, the level of the input video signal is attenuated to (R.sub.11 //R.sub.12)/(R.sub.9 //R.sub.10 +R.sub.11 //R.sub.12). When the voltage of the video signal is V.sub.C or more, its level is attenuated to (R.sub.10 //R1.sub.1 //R.sub.12)/(R.sub.9 R.sub.10 //R.sub.11 //R.sub.12). When the resistances of the resistors R.sub.9 to R.sub.12 are appropriately set, nonlinear processing such as .lambda.-correction shown in FIG. 5 and white compression processing is performed in accordance with the nonlinear regression approximation.
The level of the nonlinearly processed video signal is amplified by the amplifier 35 into an optimal input level of the A/D converter 36. For example, the amplified signal is subjected to digital signal processing (not shown).
In this conventional processing apparatus, however, (a) when the voltages V.sub.CP, V.sub.A, V.sub.B, and V.sub.C in FIG. 5 are close to each other, the response characteristics of the differential switch or variations in the reference voltages V.sub.A, V.sub.B, and V.sub.C which are caused by variations in resistances of the resistors R.sub.5 to R.sub.8 become nonnegligible, and good nonlinear characteristics cannot be obtained at points of changes in regression approximation characteristics.
(b) In order to prevent the above drawback, the video signal is sufficiently amplified by the amplifier 32 to increase differences between the reference voltages. In this case, since the video signal in image pickup signal processing has a magnitude which is several times that of a normal television signal, the dynamic range of the amplifier 32 becomes insufficient.
(c) In the conventional apparatus, since nonlinear processing has attenuation characteristics, the video signal must match the input level of the A/D converter 36, which requires a relatively large amplitude. The circuit size is undesirably increased as a result.
In the conventional apparatus, gain control processing such as white balancing of the video input signal is performed by the VCA 31. In order to improve the frequency characteristics of the VCA 31 and to simplify its circuit arrangement, reference potentials across a ladder resistor circuit in the A/D converter 36 are changed to variably control the gain of the A/D converter 36, thereby performing gain control processing.
(d) In this case, nonlinear processing must be performed prior to gain control processing such as white balancing. In the conventional processing apparatus, however, since the nonlinear characteristics are fixed, the above arrangement cannot be realized, resulting in inconvenience.